In the present day manufacture of integrated circuits, complex circuit patterns are formed on a silicon wafer by photoresist techniques employing a series of contact printings on the wafer. These contacts printings are made from several transparent masks used in succession and in a preselected order. Each successive mask must be accurately aligned with the previous print or prints made on the wafer from the prior masks so that the completed pattern is accurate within a few microns.
The alignment of each mask with the wafer may be accomplished manually by manipulation of the mask over the wafer while the operator observes the mask and wafer through a high power microscope. Alignment may be aided by the use of a pair of spaced-apart detection marks or alignment patterns, for example, crosses, bull's-eyes, or pinpoint alignment holes, formed on each of the wafer and mask, the pair of alignment patterns on the mask being arranged so that they are superimposed over and aligned with the pair of alignment patterns on the wafer when the wafer and mask are properly aligned
Apparatus has been proposed for producing alignment of the mask and wafer automatically, thus relieving the operator of this tedious task One form of such automatic apparatus is described in U.S. Pat. No. 3,497,705 issued Feb. 24, 1970, to A. J. Adler and entitled "Mask Alignment System Using Radial Patterns And Flying Spot Scanning". In that system a pair of spaced-apart radial alignment patterns on the wafer is adapted for alignment with a superimposed pair of spaced-apart radial alignment patterns on the transparent mask. The radials of the alignment patterns on the mask are uniformly angularly displaced relative to the radials of the alignment patterns on the wafer when the mask and wafer are properly aligned. A scanning system employing two flying spot scanners scans each of the two pairs of alignment patterns in a circular manner about a center point, measuring the angular distance between the successive radials encountered by the scanning beam. Error signals derived from misalignment of the radials of the alignment patterns on the mask relative to the radials of the alignment patterns on the wafer are utilized to produce relative movement in X, Y, and rotational directions to bring the two pairs of alignment into proper alignment.
Karlson et al., in U.S. Pat. No. 4,052,603, published Oct. 4, 1977, discloses a system for aligning a pattern mask and a photoresist-coated substrate which are separated by an optical element which involves a system for correcting misalignment in the x- or y-directions between the mask and the substrate by using stepper motors to move the substrate in the x-direction or in the y-direction relative to the mask. Rotational misalignment is corrected using a separate mechanism involving gears and levers to rotate the mask relative to the substrate. This apparatus involves use of two separate systems simultaneously to achieve successful alignment, and is rather complicated It would be desirable to have a system which moves the mask relative to a fixed substrate, rather than moving both the mask and the substrate relative to each other. It would also be desirable to achieve rotational motion and motion in the x-direction or in the y-direction relative to the substrate with a single mechanism, rather than with two separate mechanisms.